Ferritic and martensitic wrought Cr—Mo, Cr—Mo—V, etc. steels, introduced in the 1940s, are preferred structural materials for elevated-temperature applications. For example, such steels are used in many parts of fossil-fired power plants—from boilers to turbines. Moreover, such steels are used extensively in the petrochemical industry. The steels are also used in nuclear fission power plants and are contemplated for use in future fusion reactor plants.
Some major advantages of ferritic and martensitic, normalized-and-tempered and/or quenched-and-tempered, wrought Cr—Mo, Cr—Mo—V, etc. steels include good thermal properties, such as, for example, high thermal conductivity and low expansion coefficient, relative to other high-temperature alloys, such as the austenitic stainless steels. A shortcoming has been high-temperature tensile and creep strength, which places a limit on the upper operating temperature of the steels. This stems from the instability of the as-tempered microstructure, which includes a low-number density of sub-micron-size Cr, Nb, and/or V carbides, nitrides, and/or carbonitrides, but few, if any, nano-scale particles. Upper operating temperatures also depend on oxidation and corrosion resistance, but the higher-chromium steels would be capable of operating at higher temperatures if creep strength were higher.
Examples of early steels of this type were 2¼Cr-1Mo (ASTM Grade 22) and 9Cr-1Mo (ASTM Grade 9), which had upper-use temperatures of about 540° C. New steels have since been introduced for which the operating temperatures were increased. For example, 9Cr-1Mo was modified to produce Grade 91 (nominally Fe-9Cr-1Mo-0.25V-0.07Nb-0.05N-0.1C) by adding vanadium, niobium and nitrogen. As a result of such modifications, the upper-use temperature for steels being used in ultrasupercritical steam plants today is about 620° C. (based on ASME Code approval for pressure-vessel applications).
Currently used power-plant steels were developed based on modified 9Cr-1Mo steel, primarily by substitution of tungsten for some of the molybdenum in modified 9Cr-1Mo, although boron and more nitrogen were also utilized. These steels are typified by: NF616 (ASTM Grade 92) (Fe-9.0Cr-1.8W-0.5Mo-0.20V-0.05Nb-0.45Mn-0.06Si-0.06N-0.004B-0.07C); E911 (Fe-9.0Cr-1.0Mo-1.0W-0.20V-0.08Nb-0.40 Mn-0.40Si-0.07N-0.11C); TB12 (Fe-12.0Cr-0.5Mo-1.8W-1.0Ni-0.20V-0.05Nb-0.50 Mn-0.10Ni-0.06Si-0.06N-0.004B-0.10C); and HCM12A (ASTM Grade 122) (Fe-12.0Cr-0.5Mo-2.0W-1.0Cu-0.25V-0.05Nb-0.30Ni-0.60 Mn-0.10Si-0.06N-0.003B-0.10C). The aforementioned compositions were all developed and introduced commercially in the 1990s for 620° C. operation with a 105 h creep-rupture strength at 600° C. of 140 MPa.
There is a need for high-temperature ferritic and martensitic steels capable of operating at temperatures beyond 620° C. and as high as 650° C. One way that has been suggested to increase the temperature limit to 650° C. and higher and still maintain the inherent advantages of ferritic and martensitic steels (i.e. high thermal conductivity and low thermal expansion) is through the use of oxide dispersion-strengthened (ODS) steels. Elevated temperature strength of ODS steels is obtained through microstructures that contain a high density of small Y2O3 or TiO2 particles dispersed in a ferrite matrix. Unfortunately, production of ODS steels involves complicated and expensive powder metallurgy and mechanical alloying methods that usually involve extrusion. The directionality in the microstructure deriving from these processing methods generally produces undesirable anisotropic mechanical properties.
There is therefore a need to for high-temperature ferritic or martensitic wrought steels that can be produced by conventional steel processing methods rather than expensive powder metallurgy/mechanical alloying methods. Furthermore, with such a processing technique, it should be easier to produce a non-directional (more uniform) microstructure, which would overcome one of the inherent problems for the ODS steels. There is a need for new steel compositions and processing methods that result in steels that have properties comparable to the best ODS steels at temperatures above 620° C.; such steels would not be processed by powder metallurgy and mechanical alloying methods and thus would not be expected to be handicapped by the microstructural directionality and associated problems inherent in the production of conventional ODS steels.